In the field of graphic art, an image is printed on a printing plate using a set of color-separation films prepared from a color original by using lithographic films. In general, a color proof is manufactured from the color-separation films before the main printing (i.e., actual printing operation) so as to check on errors in the color separation process or whether color correction and the like are necessary. The color proof is demanded to realize high resolution for enabling the formation of a halftone image with high reproducibility and to have capabilities such as high processing stability. Furthermore, in order to obtain a color proof approximated to an actual printed matter, the materials used for the color proof are preferably the materials actually used for the printed matter, for example, the substrate is preferably the printing paper and the coloring material is preferably the pigment. As for the method for manufacturing the color proof, a dry process of using no developer solution is highly demanded.
For manufacturing the color proof by a dry process, a recording system of manufacturing a color proof directly from digital signals has been developed accompanying the recently widespread electronic system in the pre-printing process (pre-press field). This electronic system is developed particularly for the purpose of manufacturing a high-quality color proof and by this system, a halftone image of 150 lines/inch or more is generally reproduced. For recording a high-quality proof from digital signals, laser light capable of modulating by the digital signals and sharply focusing the recording light is used as the recording head. Accordingly, the image-forming material used therefor is required to exhibit high recording sensitivity to the laser light and high resolution for enabling the reproduction of high-definition halftone dots.
With respect to the image-forming material for use in the transfer image formation method using laser light, a heat-fusion transfer sheet is known, where a light-to-heat conversion layer capable of generating heat upon absorption of the laser light and an image-forming layer containing a pigment dispersed in a heat-fusible component such as wax or binder are provided on a support in this order (see, JP-A-5-58045 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). According to the image formation method using this image-forming material, heat is generated in the region irradiated with the laser light of the light-to-heat conversion layer and the image-forming layer corresponding to the region is fused by the heat and transferred to an image-receiving sheet stacked and disposed on the transfer sheet, whereby a transfer image is formed on the image-receiving sheet.
JP-A-6-219052 discloses a thermal transfer sheet where a light-to-heat conversion layer containing a light-to-heat conversion substance, a very thin (0.03 to 0.3 μm) thermal peeling layer and an image-forming layer containing a coloring material are provided on a support in this order. In this thermal transfer sheet, the bonding strength between the image-forming layer and the light-to-heat conversion layer bonded with an intervention of the thermal peeling layer is diminished upon irradiation with laser light and a high-definition image is formed on an image-receiving sheet stacked and disposed on the thermal transfer sheet. This image formation method using the above-described thermal transfer sheet utilizes so-called “ablation”, more specifically, a phenomenon such that a part of the thermal peeling layer in the region irradiated with the laser light is decomposed and vaporized and thereby the bonding strength between the image-forming layer and the light-to-heat conversion layer is diminished in that region, as a result, the image-forming layer in this region is transferred to an image-receiving sheet stacked on the thermal transfer sheet.
These image formation methods are advantageous in that a printing paper having provided thereon an image-receiving layer (adhesive layer) can be used as the image-receiving sheet material and a multicolor image can be easily obtained by sequentially transferring images of different colors to the image-receiving sheet. In particular, the image formation method using ablation is advantageous in that a high-definition image can be easily obtained, and therefore, this method is useful for the manufacture of DDCP or a high-definition mask image.
In recording an image with laser light, a laser light comprising multiple beams using a plurality of laser beams is recently used so as to shorten the recording time. However, if a conventional thermal transfer sheet is used for the recording with multi-beam laser light, problems are liable to arise, for example, the light-to-heat conversion layer is transferred to the transfer image formed on the image-receiving sheet to cause layer fogging, the transfer sensitivity decreases to fail in attaining sufficiently high image density, or the resolution is low.
Furthermore, the light-to-heat conversion substance such as infrared ray absorbing dye or a decomposition product thereof contained in the light-to-heat conversion layer transfers to the image-forming layer and the obtained transfer image disadvantageously has yellowish color hue.